TRPA1 is a nonselective cation channel that belongs to the superfamily of Transient Receptor Potential (TRP) ion channels. Like other family members, functional TRPA1 channels are formed by tetramerization of 4 subunits, each containing six transmembrane domains, a pore loop between transmembrane domain 5 (S5) and 6 (S6), and intracellular N- and C-termini. TRPA1 is expressed in sensory neurons and co-localized with pain markers such as TRPV1, calcitonin gene-related peptide and bradykinin receptor (Nagata, K. et al., Journal of Neuroscience 2005, 25, 4052-4061; Story, G. M. et al., Cell 2003, 112, 819-829; Corey, D. P. et al., Nature 2004, 432, 723-730; Bautista, D. M. et al., Proceedings of the National Academy of Science U.S.A. 2005, 102, 12248-12252; Jaquemar, D. et al., Journal of Biological Chemistry 1999, 274, 7325-7333). In pain models, knockdown of TRPA1 expression by gene specific antisenses prevented and reversed cold hyperalgesia induced by inflammation and nerve injury (Obata, K. et al., Journal of Clinical Investigation 2005, 115, 2393-2401; Jordt, S. E. et al., Nature 2004, 427, 260-265; Katsura, H. et al., Exploratory Neurology 2006, 200, 112-123). Furthermore, TRPA1 gene knockout resulted in impaired sensory functions and deficits in bradykinin-evoked pain hypersensitivity (Kwan, K. Y. et al. Neuron 2006, 50, 277-289; Bautista, D. M. et al. Cell 2006, 124, 1269-1282). Collectively, these data suggest that TRPA1 plays an important role in sensory functions and pain states. As a ligand-gated channel, TRPA1 can be activated by a variety of stimuli, including noxious cold, intracellular Ca2+, endogenous substances (e.g., bradykinin), pungent natural products (e.g., allyl isothiocyanate, or AITC), environmental irritants (e.g., acrolein), amphipathic molecules (e.g., trinitrophenol and chlorpromazine) and pharmacological agents (e.g., URB597) (Macpherson, L. J. et al., Current Biology 2005, 15, 929-934; Bandell, M. et al., Neuron 2004, 41, 849-857). Bradykinin activates TRPA1 indirectly through the phospholipase C pathway following binding of bradykinin to its receptors. Trinitrophenol and chlorpromazine open TRPA1 by inducing curvature or crenation in the lipid bilayer membrane (Xu, H. et al., Nat. Neurosci. 2006, 9, 628-635; Hill, K. and Schaefer, M., J. Biol. Chem. 2007, 282, 7145-7153; Niforatos, W. et al., Molecular Pharmacology 2007, 71, 1209-1216). Most recently, it was shown that TRPA1 agonists could directly interact with the channel protein. AITC and cinnamaldehyde covalently modify several cysteine and lysine residues localized in the cytoplasmic N terminus and activate the channel (Hinman, A., Chuang, H. H., Bautista, D. M., and Julius, D. Proceedings of the National Academy of Science U.S.A. 2006, 103, 19564-19568; Macpherson, L. J., Dubin, A. E., Evans, M. J., Marr, F., Schultz, P. G., Cravatt, B. F., and Patapoutian, A., Nature 2007, 445, 541-545). In addition, intracellular Ca2+ binds to the N-terminus EF-hand domains and mediates channel opening (Zurborg, S. et al., Nature Neuroscience 2007, 10, 277-279). Together these findings have revealed potential physiological roles of TRPA1, and also indicate that TRPA1 channel gating may involve different mechanisms and molecular determinants.
Thus, modulation of TRPA1 can have many industrial and therapeutic applications. For example, TRPA1 antagonists may fulfill the need in the art for new analgesic pharmaceuticals suitable for the treatment and/or prophylaxis of nociceptive and neuropathic pain in mammals, especially in humans.